The TeraByte InfraRed Delivery (TBIRD) system is a 3U payload on a 6U CubeSat launched in May 2022 which has now demonstrated space to ground links of >1 Terabyte (TB) per pass at a max data rate of 200Gbps. As a CubeSat mission, the development of the TBIRD payload was focused on low SWaP and a “rapid prototyping” approach which accepted higher risks to accelerate the schedule and reduce costs. The optomechanical design process followed standard in-house processes to develop a system that would be robust to LEO environmental loads, with a focus on the stability of the transmit (Tx) and receive (Rx) channel performance metrics. The driving requirement of maintaining 20μrad pointing error between the TX and Rx channels forced specific attention to thermal and mechanical load changes over operational conditions, which drove major design decisions. This paper describes some of engineering challenges overcome and approaches used to make TBIRD a successful program, as well as some of the tradeoffs of rapid prototyping precision optical payloads. TBIRD successfully met and exceeded the total downlink requirements listed above, with a bandwidth of 200Gbps and a total downlink of 4.8TB of information in a single pass.
KEYWORDS: Laser communications, Laser communication terminals, Design, Telecommunications, Data communications, Laser systems engineering, Optical communications, Adaptive optics, Space operations, Satellites
The Black Hole Explorer (BHEX) is a mission concept that can dramatically improve state-of-the-art astronomical very long baseline interferometry (VLBI) imaging resolution by extending baseline distances to space. To support these scientific goals, a high data rate downlink is required from space to ground. Laser communications is a promising option for realizing these high data rate, long-distance space-to-ground downlinks with smaller space/ground apertures. Here, we present a scalable laser communications downlink design and current lasercom mission results.
Since launch in May 2022, the TeraByte Infrared Delivery (TBIRD) payload on a 6U CubeSat has successfully demonstrated 100/200 Gbps laser communications and has transferred >1 TB in a pass from low Earth orbit to ground. To support the narrow downlink beam needed for high rate communications, the payload provides pointing feedback to the host spacecraft to precisely track the ground station throughout the 5-minute pass. This paper presents the on-orbit results of the pointing and tracking system for TBIRD, including initial acquisition and closed-loop tracking performance of 20-35 μrad RMS per axis. Results from on-orbit characterization of the transmit beam are also presented. Measurements of Tx/Rx alignment show stability within 20 μrad, ensuring that tracking on the uplink accurately points the downlink.
Since launch in May 2022, NASA's TeraByte Infrared Delivery (TBIRD) program has successfully demonstrated 100-Gbps and 200-Gbps laser communication downlinks from a 6U CubeSat in low-Earth orbit to a ground station. The TBIRD system operates during 5-minute passes over the ground station and has demonstrated an error-free downlink transfer of > 1 Terabyte (TB) in a single pass. This paper presents an overview of the architecture, link operations, and system performance results to date.
Space-based VLBI imaging can dramatically improve state-of-the-art astronomical radio-imaging resolution by enabling significantly longer baseline distances and eliminating atmospheric-attenuation constraints on RF carrier imaging wavelength. However, smaller space-based apertures and sensitivity constraints impose challenging recorded-data downlink-rate requirements, potentially to 256 Gbit/s. Laser communications is a promising option for realizing such highrate long-distance downlinks with modest power and aperture demands. Here, we present a scalable lasercom architecture that can enable high-rate long-distance downlinks needed for enhanced space-based VLBI imaging from geosynchronous orbit (GEO).
NASA’s Artemis II mission includes an optical communication payload, affectionately known on board as “OpCom,” which is part of NASA’s Orion Artemis II Optical Communications (O2O) demonstration. We describe the OpCom system architecture and operations concept.
The Integrated LCRD Low-Earth Orbit User Modem and Amplifier Terminal (ILLUMA-T) payload will be launched to the International Space Station (ISS) in 2023. ILLUMA-T is an optical communications payload that will make the ISS the first space-based user to communicate with NASA’s Laser Communications Relay Demonstration (LCRD). The system will support all-optical forward links up to 150 Mbps and return links up to 1 Gbps. The payload recently underwent system level Thermal VACuum (TVAC) functional testing at MIT Lincoln Laboratory. We present an overview of the payload’s TVAC functional tests and results.
The Event Horizon Explorer (EHE) is a mission concept to extend the Event Horizon Telescope via an additional space-based node. We provide highlights and overview of a concept study to explore the feasibility of such a mission. We present science goals and objectives, which include studying the immediate environment around supermassive black holes, and focus on critical enabling technologies and engineering challenges. We provide an assessment of their technological readiness and overall suitability for a NASA Medium Explorer (MIDEX) class mission.
The Terabyte InfraRed Delivery (TBIRD) program will establish a communication link from a nanosatellite in low-Earth orbit to a ground station at burst rates up to 200 Gbps. The TBIRD payload is currently in the process of integrating with the 6-U CubeSat host bus and pre-flight testing has been completed. An overview of the pointing, acquisition, and tracking system for TBIRD is provided as well as a summary of results from pre-flight testing. TBIRD relies on the spacecraft bus to implement fine pointing corrections supplied by its quad sensor at a rate of 10 Hz. The measured accuracy of pointing feedback is about 10 μrad RMS per axis. A custom optical assembly was designed for transmitter/receiver alignment stability which was measured to be within 25 μrad two-axis through environmental testing. With TBIRD feedback in the loop, single axis pointing accuracy of the downlink is predicted to be about 30 μrad RMS.
The Terabyte Infrared Delivery (TBIRD) program will establish an optical communication link from a 6U nanosatellite in low-Earth orbit to a ground station at burst rates up to 200 Gbps. The system is capable of reliable data delivery from a 2-TB storage buffer on the payload to a ground terminal in the presence of atmospheric fading. An overview of the communication architecture for TBIRD is provided as well as results from communications performance testing of the 3U lasercom payload prior to spacecraft integration. Launch is scheduled for mid-year 2022.
Free-space laser communication systems are increasingly implemented on state of the art satellites for their high-speed connectivity. This work outlines a demonstration of the Modular, Agile, Scalable Optical Terminal (MAScOT) we have developed to support Low-Earth Orbit (LEO) to deep-space communication links. In LEO, the MAScOT will be implemented on the International Space Station to support the Integrated Laser Communications Relay Demonstration (LCRD) LEO User Modem and Amplifier Terminal (ILLUMA-T) program. ILLUMA-T's overarching objective is to demonstrate high bandwidth data transfer between LEO and a ground station via a geosynchronous (GEO) relay satellite. Outside of LEO, the MAScOT will be implemented on the Artemis-II mission to demonstrate high data rate optical communications to and from the moon as part of the Optical to Orion (O2O) program. Both missions leverage the same modular architecture despite varying structural, thermal, and optical requirements. To achieve sufficient performance, the terminal relies on a nested tracking loop to realize sub-arcsecond pointing across a ±120 ° elevation and ±175° azimuth field of regard.
The Laser-Enhanced Mission Communications Navigation and Operational Services (LEMNOS) office at Goddard Space Flight Center (GSFC) manages two NASA optical communication related projects, the Orion EM-2 Optical Communications Terminal (O2O) and the Integrated Laser Communications Relay Demonstration (LCRD) Low-Earth Orbit (LEO) User Modem and Amplifier Terminal (ILLUMA-T) projects. The main goal of LEMNOS is the advancement and implementation of optical communications systems and technologies for NASA missions. The O2O mission is sponsored by NASA’s Human Exploration and Operations (HEO) Mission Directorate. The O2O project will provide optical communications capability to the Orion series of spacecraft, starting with the demonstration of operational utility on EM-2. It will be the first time a human exploration mission will rely on optical communications for its high-bandwidth link. ILLUMA-T is sponsored by the Space Communications and Navigation (SCaN) Program Office. It is destined for the International Space Station (ISS) as an external payload attached to the Japanese Experiment Module - Exposed Facility (JEM-EF). The ILLUMA-T project is developing an optical communications user terminal to demonstrate high bandwidth data transfer between LEO and the ground through the geosynchronous LCRD relay. ILLUMA-T will be the first demonstration of a LEO user of the LCRD system, pointing and tracking from a moving spacecraft at LEO to GEO satellite and vice versa, end-to-end operational utility of optical communications, and 51 Mbps forward link to ISS from ground. Both projects are collaborations between GSFC, Massachusetts Institute of Technology – Lincoln Laboratory (MIT-LL), and a number of contractors.
Free-space optical communications in space offer many benefits over established radio frequency based communication links; in particular, high beam directivity results in efficient power usage. Such a reduced power requirement is particularly appealing to small satellites with strict size, weight and power (SWaP) requirements. In the case of free-space optical communication, precise pointing, acquisition and tracking (PAT) of the incoming beam is necessary to close the communication link. Due to the narrow beam of the laser, the critical task of accomplishing PAT becomes increasingly arduous and often requires complex systems of optical and processing hardware to account for relative movement of the terminals. Recent developments in body pointing mecha- nisms have allowed small satellites to point with greater precision. In this work, we consider an approach to a low-complexity PAT system that utilizes a single quad-cell photodetector as an optical spatial sensor, and exploits the body pointing capabilities of the spacecraft to perform the tracking maneuvers, eschewing the need for additional dedicated optical hardware. We look at the PAT performance of this approach from a systems analysis viewpoint and present preliminary experimental results. In particular, we examine the implementation of the system on NASA's TeraByte InfraRed Delivery (TBIRD) demonstration.
In recent years, NASA has been developing a scalable, modular space terminal architecture to provide low-cost laser communications for a wide range of near-Earth applications. This development forms the basis for two upcoming demonstration missions. The Integrated Low-Earth Orbit Laser Communications Relay Demonstration User Modem and Amplifier Optical Communications Terminal (ILLUMA-T) will develop a user terminal for platforms in low-Earth orbit which will be installed on the International Space Station and demonstrate relay laser communications via NASA’s Laser Communication Relay Demonstration (LCRD) in geo-synchronous orbit. The Orion EM-2 Optical Communication Demonstration (O2O) will develop a terminal which will be installed on the first manned launch of the Orion Crew Exploration Vehicle and provide direct-to-Earth laser communications from lunar ranges. We describe the objectives and link architectures of these two missions which aim to demonstrate the operational utility of laser communications for manned exploration in cislunar space.
Delivery of large volumes of data from low-Earth orbit to ground is challenging due to the short link durations associated with direct-to-Earth links. The short ranges that are typical for such links enable high data rates with small terminals. While the data rate for radio-frequency links is typically limited by available spectrum, optical links do not have such limitations. However, to date, demonstrations of optical links from low-Earth orbit to ground have been limited to ~10 to ~1000 Mbps. We describe plans for NASA’s TeraByte InfraRed Delivery (TBIRD) system, which will demonstrate a direct-to-Earth optical communication link from a CubeSat in low-Earth orbit at burst rates up to 200 Gbps. Such a link is capable of delivering >50 Terabytes per day from a small spacecraft to a single small ground terminal.
Space-based optical links can, in principle, support high data rates by using power efficient communication schemes and unconstrained spectrum. In particular, direct links from low-Earth orbit (LEO) to ground have the potential to support very high rates due to the short link distances involved. In this work, we consider an architecture for LEO-to-ground links that operate at peak rates of 100+Gb=s. Such rates are routinely achieved over fiber channels using power-efficient, fiber-coupled transceivers; however, free-space systems that use these devices may need additional error control to ensure reliable communication over an atmospheric channel. We analyze the data volume, or throughput, that can be delivered by a LEO-to-ground system using fiber-coupled transceivers in conjunction with automatic repeat request (ARQ) protocols. We show that many terabytes per day can be delivered error-free from LEO to a single ground terminal for a variety of orbit and ground terminal geometries.
Free-space optical communications links have the perpetual challenge of coupling light from free-space to a detector or fiber for subsequent detection. It is especially challenging to couple light from free-space into single-mode fiber (SMF) in the presence of atmospheric tilt due to its small acceptance angle; however, SMF coupling is desirable because of the availability of extremely sensitive digital coherent receivers developed by the fiber-telecom industry. In this work, we experimentally compare three-mode and single-mode coupling after propagating through 1.6 km of free-space with and without the use of a fast-steering mirror (FSM) control loop to mitigate atmospherically induced tilt. Here, the 3-mode fiber is a 3-mode photonic lantern multiplexer (PLM) that passively couples light into three SMF outputs. With the FSM control loop active, coupling into the PLM and the SMF yielded nearly identical coupling efficiencies, as expected. Experimental results with the FSM control loop off show that coupling from free-space to PLM increases the average power received, and mitigates the negative impacts of tilt-induced fading relative to coupling directly to SMF.
Space-based optical communication systems that transmit directly to Earth must provision for changing conditions such as received power fluctuations that can occur due to atmospheric turbulence. One way of ensuring error-free communication in this environment is to introduce link-layer feedback protocols that use an Earth-toSpace uplink to request retransmission of erroneous or missing frames. In this paper, we consider near-Earth systems that use low-bandwidth uplinks to supply feedback for automatic repeat request (ARQ) protocols. Constraining the uplink signaling bandwidth can reduce the complexity of the space terminal, but it also decreases the efficacy of feedback schemes. Using a Markov-based model of the link-layer channel, we give an analytical result for the downlink performance penalty of a system employing a data-rate-constrained selective-repeat ARQ protocol. We find that the tradeoff between downlink performance and feedback rate is primarily influenced by the coherence time of the atmospheric channel.
KEYWORDS: Atmospheric propagation, Receivers, Free space optics, Atmospheric optics, Free space optical communications, Digital signal processing, Free space, Signal to noise ratio, Composites, Telecommunications, Adaptive optics, Signal processing
The next generation free-space optical communications infrastructure will need to support a wide variety of space-to-ground links. As a result of the limited size, weight, and power on space-borne assets, the ground terminals need to scale efficiently to large collection areas to support extremely long link distances or high data rates. Recent advances in integrated digital coherent receivers enable the coherent combining (i.e., full-field addition) of signals from several small apertures to synthesize an effective single large aperture. In this work, we experimentally demonstrate the coherent combining of signals received by four independent receive chains after propagation through a 3:2-km atmospheric channel. Measured results show the practicality of coherently combining the four received signals via digital signal processing after transmission through a turbulent atmosphere. In particular, near-lossless combining is demonstrated using the technique of maximal ratio combining.
KEYWORDS: Forward error correction, Signal to noise ratio, Digital signal processing, Receivers, Modulation, Telecommunications, Free space optical communications, Transmitters, Data communications, Binary data
The next generation free-space optical (FSO) communications infrastructure will need to support a wide range of links from space-based terminals in low Earth orbit, geosynchronous Earth orbit, and deep space to the ground. Efficiently enabling such a diverse mission set requires an optical communications system architecture capable of providing excellent sensitivity (i.e., few photons-per-bit) while allowing reductions in data rate for increased system margin. Specifically, coherent optical transmission systems have excellent sensitivity and can trade data rate for system margin by adjusting the modulation format, the forward error correction (FEC) code rate, or by repeating blocks of channel symbols. These techniques can be implemented on a common set of hardware at a fixed system baud rate. Experimental results show that changing modulation formats between quaternary phase-shifted keying and binary phase-shifted keying enables a 3-dB scaling in data rate and a 3.5-dB scaling in system margin. Experimental results of QPSK transmission show a 5.6-dB scaling of data rate and an 8.9-dB scaling in system margin by varying the FEC code rate from rate-9/10 to rate-1/4. Experimental results also show a 45.6-dB scaling in data rate over a 41.7-dB range of input powers by block-repeating and combining a pseudorandom binary sequence up to 36,017 times.
Space systems operating in low-Earth orbit are often constrained by how much data can be delivered from space to ground. Traditional data delivery approaches are often limited by either large link losses associated with transmission via a geosynchronous relay satellite or short contact times and spectrum-constrained data rates associated with direct-to-Earth radio-frequency links. Direct-to-Earth optical communication links from low-Earth orbit based on fiber telecommunications technologies that can operate at high data rates (> 100 Gb/s per wavelength channel) can enable the delivery of extremely large volumes of data from space to ground. We analyze the performance of such systems and discuss the performance gains that are enabled by coupling the received signal to an efficient single-mode-fiber-based receiver, even in the presence of turbulence-induced losses.
KEYWORDS: Receivers, Free space optics, Digital signal processing, Optical communications, Free space optical communications, Transmitters, Signal to noise ratio, Clocks, Modulation, Binary data, Modulators
The next generation free-space optical (FSO) communications infrastructure will need to support a wide range of links from space-based terminals at LEO, GEO, and deep space to the ground. Efficiently enabling such a diverse mission set requires a common ground station architecture capable of providing excellent sensitivity (i.e., few photons-per-bit) while supporting a wide range of data rates. One method for achieving excellent sensitivity performance is to use integrated digital coherent receivers. Additionally, coherent receivers provide full-field information, which enables efficient temporal coherent combining of block repeated signals. This method allows system designers to trade excess link margin for increased data rate without requiring hardware modifications. We present experimental results that show a 45-dB scaling in data rate over a 41-dB range of input powers by block-repeating and combining a PRBS sequence up to 36,017 times.
KEYWORDS: Navigation systems, Laser communications, Telecommunications, Data communications, Satellites, Clocks, Phase measurement, Receivers, Space operations, Global Positioning System
The Lunar Laser Communication Demonstration (LLCD) flown on the Lunar Atmosphere and Dust Environment Explorer (LADEE) satellite achieved record uplink and downlink communication data rates between a satellite orbiting the Moon and an Earth-based ground terminal. In addition, the high-speed signals of the communication system were used to accurately measure the round-trip time-of-flight (TOF) of signals sent to the Moon and back to the Earth. The measured TOF data, sampled at a 20-kS/s rate, and converted to distance, was processed to show a Gaussian white noise floor typically less than 1 cm RMS. This resulted in a precision for relative distance measurements more than two orders-of-magnitude finer than the RF-based navigation and ranging systems used during the LADEE mission. This paper presents an overview of the LLCD TOF system, a summary of the on-orbit measurements, and an analysis of the accuracy of the measured data for the mission.
The Lunar Laser Communication Demonstration (LLCD) successfully demonstrated for the first time duplex laser communications between a lunar-orbiting satellite and ground stations on Earth with error-free downlink data rates up to 622 Mb/s utilizing an optical receiver based on photon-counting superconducting nanowires and operating near 1550 nm.
KEYWORDS: Space operations, Space telescopes, Laser communications, Telescopes, Data communications, Clouds, Time division multiplexing, Video, Multiplexing, Laser systems engineering
From mid-October through mid-November 2013, NASA’s Lunar Laser Communication Demonstration (LLCD) successfully demonstrated for the first time duplex laser communications between a satellite in lunar orbit, the Lunar Atmosphere and Dust Environment Explorer (LADEE), and ground stations on the Earth. It constituted the longest-range laser communication link ever built and demonstrated the highest communication data rates ever achieved to or from the Moon. The system included the development of a novel space terminal, a novel ground terminal, two major upgrades of existing ground terminals, and a capable and flexible ground operations infrastructure. This presentation will give an overview of the system architecture and the several terminals, basic operations of both the link and the whole system, and some typical results.
KEYWORDS: Space operations, Space telescopes, Telescopes, Laser communications, Atmospheric optics, Optical communications, Receivers, Data communications, Free space optical communications, Electronics
The Lunar Laser Communication Demonstration will be NASA's first attempt to demonstrate optical communications from a lunar orbiting spacecraft to an Earth-based ground receiver. A low SWAP optical terminal has been built and integrated onto the Lunar Atmosphere and Dust Environment Explorer (LADEE) spacecraft, presently scheduled to launch in 2013. LLCD will demonstrate duplex optical communications between this small space terminal and a multi-aperture photon-counting ground terminal at downlink data rates of up to 622 Mbps and uplink data rates of up to 20 Mbps. The system will also perform two-way time-of-flight measurements with the potential to perform ranging with sub-centimeter accuracy. As of the time of this conference, the Lincoln-built ground terminal has been constructed at a temporary site near Lincoln Lab nnd the two alternate ground terminals – being built by JPL and ESA – are in preparation.
Improvements to a ground-based 40W 1.55 micron uplink transmitter for the Lunar Laser Communications
Demonstration (LLCD) are described. The transmitter utilizes four 10 W spatial-diversity channels to broadcast 19.4 -
38.9 Mbit/s rates using a variable-duty cycle 4-ary pulse position modulation. At the lowest rate, with a 32-to-1 duty
cycle, this leads to 320 W peak power per transmitter channel. This paper discusses a simplification of the transmitter
that uses super-large-area single mode fiber and polarization control to mitigate high peak power nonlinear impairments.
We discuss the use of photon-counting array receivers for communications links employing on-off-keyed and frequencyshift-
keyed modulation formats. The effects of detector non-idealities, such as reset time and timing resolution on
achievable receiver performance are presented.
The Lunar Laser Communications Demonstration (LLCD), a project being undertaken by
MIT Lincoln Laboratory, NASA's Goddard Space Flight Center, and the Jet Propulsion
Laboratory, will be NASA's first attempt to demonstrate optical communications between a
lunar orbiting spacecraft and Earth-based ground receivers. The LLCD space terminal will
be flown on the Lunar Atmosphere and Dust Environment Explorer (LADEE) spacecraft,
presently scheduled to launch in 2013. LLCD will demonstrate downlink optical
communications at rates up to 620 Mbps, uplink optical communications at rates up to 20
Mbps, and two-way time-of-flight measurements with the potential to perform ranging with
sub-centimeter accuracy.
KEYWORDS: Space operations, Space telescopes, Telescopes, Receivers, Optical communications, Laser communications, Transmitters, Electronics, Optical fibers, Control systems
The Lunar Laser Communications Demonstration (LLCD), a project being undertaken by MIT Lincoln Laboratory and
NASA's Goddard Space Flight Center, represents NASA's first attempt to demonstrate optical communications from a
lunar orbiting spacecraft to an Earth-based ground receiver. The LLCD space terminal will be flown on the Lunar
Atmosphere and Dust Environment Explorer (LADEE) spacecraft, presently scheduled to launch in 2013. LLCD will
demonstrate downlink optical communications at rates up to 620 Mbps, uplink optical communications at rates up to 20
Mbps, and two-way time-of-flight measurements with the potential to perform ranging with sub-centimeter accuracy.
We describe the objectives of the LLCD program and discuss key technologies employed in the space and ground
terminals.
The space terminal modem for the Lunar Laser Communications Demonstration (LLCD) provides duplex
lasercom capabilities between the Earth and a satellite in lunar orbit with a 0.5-W optical transmitter
delivering downlink data rates of 39-620 Mbps and an optically-preamplified direct detection receiver
supporting uplink data rates of 10-19 Mbps. The modem consists of four subsystem modules: digital
electronics, analog electronics, power conditioning, and electro-optics. This modular approach permits
subsystems to be built and tested in parallel and provides design flexibility to address evolving
requirements. Other important design considerations for the modem include the utilization of commercial-off-
the-shelf (COTS) components to reduce delivery time, cost, minimization of size, weight, and power,
and the ability to survive launch conditions and operate over a broad temperature range in lunar orbit.
Superconducting nanowire single photon detectors have recently been demonstrated as viable candidates for photon-counting
optical receivers operating at data rates in excess of 100 Mbit/s. In this paper, we discuss techniques for
extending these data rates to rates > 1 Gbit/s. We report on a recent demonstration of a 2-element nanowire detector
array operating at a source data rate of 1.25 Gbit/s. We also describe techniques for emulating larger arrays of detectors
using a single detector. We use these techniques to demonstrate photon-counting receiver operation at data rates from
780-Mbit/s to 2.5 Gbit/s with sensitivities ranging from 1.1 to 7.1 incident photons per bit.
Silicon Geiger-mode avalanche photodiodes (Si GM-APDs) have desirable properties for an optical
photon-counting receiver, including high single-photon detection efficiency, low reset time, and low
timing jitter; however, they do not detect near-IR photons. In this work, we demonstrated a sensitive
photon-counting receiver in the near-IR by combining a wavelength converter consisting of a
periodically-poled lithium niobate (PPLN) waveguide and a commercial Si GM-APD detector. We
measured a receiver sensitivity from 1.4 to 3.5 incident photons/bit from 5.5 Mb/s to 22 Mb/s for a
single detector, and achieved a sensitivity of 4 photons/bit at 78 Mb/s using an emulated array of 25
detectors.
The sensitivity of a high-rate photon-counting optical communications link depends on the performance of the photon counter used to detect the optical signal. In this paper, we focus on ways to reduce the effect of blocking, which is loss due to time periods in which the photon counter is inactive following a preceding detection event. This blocking loss can be reduced by using an array of photon counting detectors or by using photon counters with a shorter inactive period. Both of these techniques for reducing the blocking loss can be employed by using a multi-element superconducting nanowire single-photon detector. Two-element superconducting nanowire single-photon detectors are used to demonstrate error-free photon counting optical communication at data rates of 781 Mbit/s and 1.25 Gbit/s.
Ultrafast optical time-division multiplexing (OTDM) networks have the potential to provide truly flexible bandwidth-on-demand at burst rates in excess of 100 Gbit/s for high-end users, high-speed video servers, terabyte media banks, supercomputers, and aggregates of lower speed users. Because 100 Gbit/s channel rates exceed the current speed available from electronics, functions such as slot or packet synchronization, header address comparison, and data rate conversion at OTDM packet routers or network receiver nodes must be achieved using all-optical techniques. Interferometric logic gates based on gain and index nonlinearities in semiconductor optical amplifiers (SOAs) are of particular interest due to their compact size, low latency, low required switching pulse energies, and potential for large-scale integration. One challenge for SOA-based optical switching is gain saturation that leads to pattern-dependent amplitude modulation at the switch output. We demonstrate pulse-position modulation as a viable means for mitigating carrier-induced amplitude patterning and use this data format to implement optical switches capable of stable operation at 100 Gbit/s data rates with low switching energies. We also show that semiconductor-based optical logic gates can be cascaded together to achieve advanced functionality for ultrafast system applications. As an example, we will present our recent implementation of a synchronous OTDM network testbed capable of fully loaded packet transmission. We demonstrate receiver functionality with multi-layered independent all-optical logic to achieve packet self-synchronization, multiple-bit address comparison, and data demultiplexing at channel speeds exceeding 100 Gbit/s.
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